PhotosynthesisThe leaf is the main photosynthetic structure. The chloroplasts are the cellular organelles withinthe leaf where photosynthesis takes place.Structure of the leafLeaves are adapted to bring together the three raw materials ofphotosynthesis (water, carbon dioxide and light) and remove itsproducts (oxygen and glucose). These adaptations include: A large surface area that collects as much sunlight as possible An arrangement of leaves on the plant that minimises overlapping and so avoids the shadowing of one leaf by another Thin, as most light is absorbed in the first few millimetres of the leaf and the diffusion distance is thus kept short A transparent cuticle and epidermis that let light through to the photosynthetic mesophyll cells beneath Long, narrow upper mesophyll cells packed with chloroplasts that collect sunlight Numerous stomata for gaseous exchange Stomata that open and close in response to changes in light intensity Many air spaces in the lower mesophyll layer to allow diffusion of carbon dioxide and oxygen A network of xylem that brings water to the leaf cells, and phloem that carries away the sugars produced in photosynthesisStructure and role of chloroplastsChloroplasts vary in shape and size but are typically disc shaped, 2-10µm long and 1µm indiameter. They are surrounded by a double membrane and have two distinct regions within themembrane: The grana are stack of up to 100 disc-like structures called thylakoids where the light-dependent stage of photosynthesis takes place. Within the thylakoids is the photosynthetic pigment called chlorophyll. Some thylakoids have tubular extensions that join up with thylakoids in adjacent grana; these are called inter-granal lamellae. The stroma is a fluid-filled matrix where the light-independent stage of photosynthesis takes place. Within the stroma are a number of other structures such as starch grains.The overall (simplified) equation for photosynthesis is:6CO2+ 6H C 2O 6H 12O 6+ 6O2There are three main stages to photosynthesis: 1. Capturing of light energyby chloroplast pigments such as chlorophyll

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The light-dependent reaction , in which light energy is converted into chemical energy 3. The light-independent reaction , in which protons (hydrogen ions) from the LDR are used to reduce carbon dioxide to produce sugars and other organic moleculesThe light-dependent reactionThe LDR of photosynthesis involves the capture of light whose energy is used for two purposes: To add an inorganic phosphate molecule to ADP, thereby making ATP. To split water into H+ ions and OH-ions.…read more

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The light-independent reactionThe products of the LDR (namely ATP and NADPH) are used to reduce carbon dioxide in thesecond stage of photosynthesis. Unlike the first stage, this stage does not require light directlyand, in theory, occurs whether or not light is available. In practice, however, it requires theproducts of the LDR and so rapidly ceases when light is absent. The LIR takes place in the stroma ofthe chloroplasts and is referred to as the Calvin cycle. 1.…read more

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Factors affecting photosynthesisIn a complex process such as photosynthesis, the factors that affect it all operate together.However, the rate of photosynthesis at any given moment is not affected by all the factors, butrather by the one whose level is at the least favourable value. This is the limiting factor because itlimits the rate at which the process can take place.Effect of light intensityWhen light is the limiting factor, the rate of photosynthesis is directly proportional to lightintensity.…read more